Blazar Sheath Illumination of the Outer Molecular Torus: A Resolution of the Seed Photon Problem for the far-GeV Blazar flares

TL;DR

This paper proposes that the sheath's IR radiation from molecular clouds around the torus provides seed photons for GeV blazar flares, resolving the seed photon problem at several parsecs from the core.

Contribution

It introduces a model where sheath-illuminated dust clouds supply seed photons for leptonic inverse Compton emission in distant blazar jets.

Findings

Sheath IR radiation can dominate seed photon field at several parsecs.

Dust sublimation occurs if sheath luminosity is high enough.

Optical-UV flares can illuminate dust clouds, producing correlated emission lines.

Abstract

Recent multi-wavelength work led by the Boston University blazar group (e.g., Marscher et al. 2010) strongly suggests that a fraction of the blazar flares seen by the Fermi Large Area Telescope (LAT) take place a few to several pc away from the central engine. However, at such distances from the central engine, there is no adequate external photon field to provide the seed photons required for producing the observed GeV emission under leptonic inverse Compton (IC) models. A possible solution is a spine-sheath geometry for the emitting region (MacDonald et al. 2015, but see Nalewajko et al. 2014). Here we use the current view of the molecular torus (e.g., Elitzur, 2012; Netzer 2015) in which the torus extends a few pc beyond the dust sublimation radius with dust clouds distributed with a declining density for decreasing polar angle. We show that for a spine-sheath blazar jet embedded in the torus, the wide beaming pattern of the synchrotron radiation of the relatively slow sheath will heat molecular clouds whose subsequent IR radiation will be seen highly boosted in the spine comoving frame, and that under reasonable conditions this photon field can dominate over the sheath photons directly entering the spine. If the sheath is sufficiently luminous it will sublimate the dust, and if the sheath synchrotron radiation extends to optical-UV energies (as may happen during flares), this will illuminate the sublimated dust clouds to produce emission lines that will vary in unison with the optical-UV continuum, as has been very recently reported for blazar CTA 102 (Jorstad et al. 2017).